from collections import deque
graph = {0: [1,2], 1: [0,3], 2: [0,3], 3: [1,2]}
queue = deque([0])
visited = []
seen = {0}
while queue:
    node = queue.popleft()
    visited.append(node)
    for n in sorted(graph[node]):
        if n not in seen:
            seen.add(n)
            queue.append(n)
print(visited)